Method of producing bright surfaces on aluminum



Patented May 12, 1936 UNITED STATES METHOD OF PRODUCING BRIGHT SURFACES ON ALUMINUM Ralph B. Mason, New Kensington, and Martin Tosterud, Arnold, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa., a. cornotation of Pennsylvania no Drawing. Application February 10, 1934,

Serial No. 710,723

16 Claims.

This invention relates to bright aluminum surfaces, and it is particularly concerned with a method for the treatment of aluminum articles to produce thereon permanent bright surfaces of high reflectivity.

Bright aluminum surfaces have found extensive application for decorative purposes and particularly for the reflection of radiant energy of various kinds such as visible light. These bright aluminum surfaces may be produced either by etching for the production of diffusely reflecting surfaces or by mechanical bufflng or polishing to produce specular surfaces. This invention is concerned primarily with a method of increasing and preserving the brightness of such aluminum surfaces, and particularly their light reflecting power.

It is an object of the invention to provide a method for the treatment of an aluminum sur- 20 face whereby the reflection factor of such surface is increased and to provide such a surface with a protective transparent coating. More particularly it is an object of the invention to provide a method for the treatment of aluminum surfaces to remove impurities therefrom and to provide such surface with a thin transparent protective coating. Other objects of the invention will become apparent from the following description.

Various attempts have been made in the past to provide aluminum reflecting surfaces. with transparent protective coatings for the purpose of preserving their reflecting power, and of making them more durable under severe service conditions. Attempts have been made to provide such transparent protective coatings by anodic oxidation in various electrolytes, such as, for example, sulfuric acid solutions. In general, however, these attempts have been unsuccessful because the oxide coatings, when produced in suflicient thickness to afford substantial protection, appeared more or less translucent or foggy, and the reflectivity of the treated surface was substantially decreased. Impurities in the aluminum, and dirt, grease or other foreign matter held on the aluminum surface are apparently the cause of this translucence and fogginess in the oxide coating.

We have now found that by the addition of hydrofluoric acid to sulfuric acid electrolytes, the normal oxidizing action of such electrolytes on aluminum made anode therein may be modified to produce combined brightening and coating action, and that by anodically treating an aluminum article in an electrolyte containing sulfuric acid and hydrofluoric acid such impurities and forin and on the aluminum surface. This property sulfuric acid will impart the desired properties eign matter as may be present on the aluminum surface may be removed and a thin substantially transparent protective film simultaneously produced on the surface. There is thus obtained a more durable reflecting surface generally having an increased reflection factor. Furthermore, the bright coated aluminum reflecting surface so obtained may be further treated to increase its durability by anodic oxidation without seriously impairing its reflection factor. Other forms of protective coatings, such as clear lacquer or varnish coatings, may also be applied to the bright, durable reflecting surface obtained by the method of this invention if desired.

The addition of hydrofluoric acid to the sulfuric acid electrolyte modifies the normal electrochemical action of the sulfuric acid on the aluminum surface by decreasing its oxide coatforming properties and imparting to the electrolyte the property of removing impurities present of removing impurities is apparently due to an increase in the solvent action of the electrolyte. This solvent action is, however, so uniform under the influence of the applied electric current that even when a polished specular aluminum reflecting surface is treated, according to the invention, as anode in a sulfuric acid electrolyte containing a small amount of hydrofluoric acid, the surface is cleaned and brightened with an increase in the reflection factor depending upon the amount of impurities and dirt removed from the surface, and at the same time the specular characteristics of the surface are not materially modified.

Hydrofluosilicic acid and other fluorine compounds which yield hydrofluoric acid when in solution with sulfuric acid, such as the fluoride salts, may be substituted for hydrofluoric acid in equivalent amounts for the purposes of this invention;

The minimum effective amount of hydrofluoric acid necessary to impart to the sulfuric acid electrolytes the desired properties, varies somewhat. Very small amounts of hydrofluoric acid in the in some degree. In general, however, it is considered desirable to have present in the electrolyte not less than 0.2 per cent of HF by weight. With increased amounts of hydrofluoric acid present, the possibility of direct chemical attack on the aluminum surface by the electrolyte is increased, and it is necessary to increase the current density of the-applied electric current to prevent this direct chemical attack which tends to produce irregularities in the reflecting surface. while, therefore, higher concentrations of hydrofluoric acid may be used with high ,current densities, it is generally not desirable however, to use electrolytes containing about 5 to 35 per cent sulfuric acid, and very good results are obtainable with electrolytes containing 25 per cent by weight sulfuric acid.

- In can'ying out the electrolytic treatment, it is preferable that the temperature of the electrolyte be somewhat above room temperature,

and temperatures between about 30 and 70 C."

have proved most satisfactory. With operation of the bath at these temperatures, satisfactory results can generally be obtained by a treatment for 5 to 30 minutes. In most cases it is preferred to treat at a temperature of about 50 C. for about 10 minutes.

This electrolytic brightening treatment may be carried out either with direct or alternating current. The current density used will vary with the exact composition of the electrolyte and the temperature of operation. The current density should be sufficient to prevent any direct chemical attack of the electrolyte on the aluminum surface and this current density should not be substantially exceeded. With greater current densities the modifying effect of the hydrofluoric acid on the oxide coat-forming properties of the sulfuric acid is lessened. In general higher current densities are required for operation at the higher temperatures. When using direct current with electrolytes containing 1 to per cent sulfuric acid and 0.2 to 1.5 per cent hydrofluoric acid at temperatures of about 30 to 70 C., satisfactory results are obtainable with current densities in the range of about 10 to 100 amperes per square foot of anode surface. When using alternating current, somewhat higher current densities are used than are required with direct current for corresponding electrolytes at corresponding temperatures.

Prior to the treatment of an aluminum reflecting surface according to this invention it is sometimes desirable, in order to obtain the best results, to preliminarily treat the aluminum surface to remove superficial dirt and grease such as may be present thereon as a result of a previous polishing operation. Any convenient method of preliminary cleaning may be employed, preferably by washing the surface with a solvent which does not objectionably attack the metal and which does not require such rubbing of a polished surface as may tend to injure its appearance.

Subsequent to the electrolytic treatment of the aluminum reflecting surface in the sulfuric acid-hydrofluoric acid electrolyte the reflecting surface may be anodically oxidized to form thereon a comparatively dense oxide coating which is substantially transparent. By the term oxide coating as used herein and in the appended claims is meant such coatings concurrently so designated in the art which consist in substantial part of aluminum oxide. It is desirable that the oxide coatings produced be substantially colorless, clear and transparent in order that, the reflectivity of the brightened aluminum reflecting surface will be reduced as little as possible. While any electrolyte capable of producing such coatings may be used, it has been found that sulfuric acid and oxalic acid solutions are particularly suitable for this purpose. The amount .of reduction in reflectivity caused by oxide coatticularly when the step of treating in hot water.

has been employed, a very light polishing with a mildly abrasive material such as magnesia, silver polish, or a mildly abrasive soap powder may be desirable to remove any superficial deposit which.

may have been formed on the oxide-coated reflecting surface by reason of any of the previous operations.

The following examples of speciflc methods of carrying out the method of this invention show the advantages resulting therefrom.

An aluminum article having a high purity aluminum surface was buffed to produce a specular reflecting surface which had a reflection factor of 75.5 per cent. The article was then made the anode in an electrolytic cell containing an electrolyte formed by the addition of 2 per cent of commercial hydrofluoric acid containing 48 per cent HF to a solution of sulfuric acid containing 25 per cent by weight H2304. A direct current, at a current density of about 92 amperes per square foot was employed at a potential of about 8 volts for 10 minutes, the electrolyte being maintained at a temperature of about 60 C. The reflecting surface, after this treatment. had a reflection factor of 87.0 .per cent. The article was then anodically oxidized in a 12 per cent sulfuric acid solution at a temperature of 24 C., using a current density of about 12 amperes per square foot anda potential of 16 volts for about 10 minutes. The oxide-coated reflecting surface thus obtained wastreated with pure boiling water for about 10 minutes, and was polished with a mildly abrasive soap powder. The reflection factor of the protected reflecting surface thus obtained was about 84.5 per cent. The article could be handled without permanent marking or staining and could be readily washed or wiped without depreciation of its reflecting power.

Another similar article having a high purity buffed aluminum surface which had a reflection factor of 75.2 per cent was made an electrode in an electrolytic cell, the electrolyte of which was formed by the addition, of 2 per cent of commercial hydrofluoric acid, containing 48 per cent HF, to a 25 per cent solution of sulfuric acid. An alternating current having a current density of about 80 amperes per square foot was employed at a potential of about 10 volts for 20 minutes,

oxidized in a 7 per cent sulfuric acid electrolyte for a period of 10 minutes at a current density of 12 amperes per square foot and a potential of 22 volts, while the electrolyte was maintained at a temperature of about 22 C. The oxide-coated article was then treated with pure boiling water for 10 minutes and was flnally polished with a mildly abrasive soap powder. The reflection factor of the completed article was then about 83.4 per cent.

In measuring'the light reflection factors of the surfaces above referred to, the Taylor reflectometer devised by A. H. Taylor of the National Bureau of Standards and described in the Scientiflc Papers of the Bureau of Standards Nos. 8-391 and 405 was used.

In general aluminum alloy reflecting surfaces, when treated according to the invention in sulfuric acid-hydrofluoric acid electrolyte and subsequently anodically oxidized, do not have as high reflection factors as are obtainable by the same treatment on a high purity aluminum surface. However, the method herein described is applicable to many aluminum base alloys with advantage, and the term aluminum" as used throughout this specification is to be understood to include both aluminum and aluminum base alloys.

We claim:

1. A method of brightening aluminum surfaces and simultaneously producing thereon a clear, transparent film, comprising removing dirt and impurities from said surface by subjecting said surface to an electrolytic treatment in a solution containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid.

2. A method of brightening aluminum surfaces and simultaneously producing thereon a transparent film, comprising removing dirt and impurities from said surface by treating said surface anodically in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid.

3. A method of brightening aluminum surfaces and simultaneously producing thereon a clear, transparent film, comprising removing dirt and impurities from said surface by subjecting said surface to an electrolytic treatment in an electrolyte containing 1 to 60 per cent by weight of sulfuric acid and 0.2 to 1.5 per cent by weight of BF.

4. A method of brightening aluminum surfaces and simultaneously producing thereon a transparent film, comprising removing dirt and impurities from said surface by treating said surface anodically in an electrolyte containing 1 to 60 per cent by weight of sulfuric acid and 0.2 to 1.5 per cent by weight of HF.

5. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid and thereafter producing on said surface a clear, transparent, impermeable coating.

6. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid and thereafter producing on said surface, by anodic oxidation, 9.

clear, transparent coating consisting substantially of aluminum oxide.

7. A method of producing an aluminum article having a durable, bright surface, comprising ancdically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid and 0.2 to 1.5 per cent HF by weight and thereafter producing on said surface a clear, transparent, impermeable coating consisting substantially of aluminum oxide.

8. A method of producing an aluminum article having a durable, bright surface, comprising anodically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid and thereafter producing on said surface, by anodic oxidation in sulfuric acid, a clear, transparent coating consisting substantially of aluminum oxide.

9. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and hydrofluoric acid, and thereafter producing on said surface, by anodic oxidation, a clear, transparent coating consisting substantially of aluminum oxide, and impermeabilizing the oxide-coated surface by treating with hot water.

10. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight, and thereafter producing on said surface, by anodic oxidation, a clear, transparent coating consisting substantially of aluminum oxide, and impermeabilizing the oxide-coated surface by treating with water at to C.

11. A method of producing a specular, highly reflecting surface on aluminum, comprising removing dirt and impurities from a polished a1uminum reflecting surface by treating said surface electrolytically in a solution containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight at a current density sufflcient to substantially prevent direct chemical attach of the electrolyte on the aluminum surface.

12. A method of producing a durable, specular, reflecting surface on aluminum, comprising removing dirt and impurities from a polished aluminum reflecting surface by treating said surface electrolytically in a solution containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight at a current density sufficient to substantially prevent direct chemical attack of the electrolyte on the aluminum surface, and thereafter anodically oxidizing the clean, bright surface obtained.

13. A method of producing a durable, specular, reflecting surface on aluminum, comprising removing dirt and impurities from a polished aluminum reflecting surface by treating said surface electrolytically in asolution containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight at a current density sufficient to substantially prevent direct chemical attack of the electrolyte on the aluminum surface, thereafter anodically oxidizing the clean, bright surface obtained, and treating the oxide-coated surfaces with water at 80 to 100 C.

14. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight, and thereafter producing on said surface a clear, transparent, impermeable coati 15. A method of producing an aluminum article having a durable, bright surface, comprising electrolytically treating the aluminum article in an electrolyte containing 1 to 60 per cent sulfuric acid by weight and 0.2 to 1.5 per cent HF by weight, and thereafter producing on said surface, by anodic oxidation, a clear, transparent coating consisting substantially of aluminum oxide.

16. A method of producing an aluminum arsurface, by anodic oxidation, 9. clear, transparent ticle having a durable, bright surface, comprlscoating consisting substantially of aluminum oxing electrolytically treating the aluminum artiide and impermeablizing the oxide-coated surcle in an electrolyte containing 1 to 60 per cent face by treating with hot water.

5 sulfuric acid by weight and 02 to 1.5 per cent RALPH B. MASON.

HF by weight, and thereafter producing on said MARTIN TOSTERUD.

CERTIFICATE OF CORRECTION.

Patent No. 2,040,618. May 12, 1936.

RALPH B. MASON, ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, first column, line 59, claim 6, after "aluminum" insert the word article; and second column, line 34. claim 11 for "attach" read attack; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 30th day of June, A. D. 1936.

- Henry Van Arsdalo (Seal) Acting Commissioner of Patents. 

